If your Firefox window shakes/jitters/bounces, try the following steps:
If your Bookmarks Toolbar is showing but it does not contain any bookmarks, add a bookmark to it or hide it (View -> Toolbars -> uncheck Bookmarks Toolbar)
If you put your Bookmarks Toolbar Items on a different toolbar (such as the File/Edit menubar) and have hidden the Bookmarks Toolbar itself, remove the Bookmarks Toolbar Items (View -> Toolbars -> Customize -> drag the Bookmarks Toolbar Items back to the Customize Toolbar window -> click Done). Now, replace the Bookmarks Toolbar Items to where you want it.
If you recently switched to a different Firefox theme, switch back to the default one (Tools -> Themes -> Firefox (default) -> Use Theme)
If you are using a non-standard OS theme, try switching back to the default one
